The present disclosure relates to the subject matter disclosed in international application No. PCT/EP00/10994 of Nov. 8, 2000, which is incorporated herein by reference in its entirety and for all purposes.
The invention relates to a transportable cooling unit for maintaining a transport volume at the defined temperature, comprising a closed cooling circuit serially including a multi-stage compressor, a condenser, an expansion device and an evaporator arranged in said transport volume as well as a speed-controlled electric motor driving said compressor.
Such transportable cooling units are, for example, disclosed in the article of R. D. Heap xe2x80x9cRefrigerated containers in . . . xe2x80x9d.
The object of the present invention is to provide a transportable unit cooling which provides minimized energy consumption in combination with optimized temperature stability within the transport volume irrespective of the environment.
This object is achieved by a transportable cooling unit for maintaining a transport volume at a defined temperature, comprising a closed cooling circuit serially including a multi-stage compressor, a condenser, an expansion device and an evaporator arranged in said transport volume, a speed-controlled electric motor driving said compressor and a controller sensing a temperature present within said transport volume and controlling said electric motor so as to provide the cooling power demanded at said evaporator for maintaining said defined temperature and minimize energy consumption, said controller operating said closed cooling circuit between a minimum possible cooling power and a maximum possible cooling power in a sequence of different operational stages comprising a lowest operational stage (stage 1) and a sequence of at least two upper operational stages (stage 2 to 4), said controller operating said closed cooling circuit in each one of said upper operational stages (stage 2 to 4) at a compressor speed related cooling capacity different from said compressor speed related cooling capacity in said other upper operational stages and within said respective upper operational stages in an uninterrupted mode and adjusting said cooling power provided by said closed cooling circuit by an essentially stepless speed control of said electric motor.
The advantage of the present invention is to be seen in the fact that due to the sequence of different operational stages the compressor can be run within a reasonable speed range which is advantageous for an optimized compressor design and optimized compressor energy consumption but within the speed range different levels of cooling power can be achieved by using different operational stages of the closed cooling circuit, which makes it possible to minimize energy consumption of the entire system.
According to the present invention control of the speed of the electric motor could be achieved by various means. It turned out to be advantageous for the speed controllable electric motor to be a frequency controlled AC-motor, because in such a frequency controlled AC-motor the energy consumption can be reduced in accordance with the speed of the controllable electric motor.
In accordance with the aforementioned definitions of the present invention it is not defined how the controller operates the compressor in said low operational stage. It is particularly advantageous if in said lowest operational stage said controller operates said compressor in an interrupted mode at low speed and adjusts said necessary cooling capacity by adjusting at least one of the parameters comprising interruption interval and speed.
The advantage of this embodiment of the present invention is that in the low operational stage it is allowed to switch the compressor on and off so as to be able to control low cooling power of the closed cooling circuit and to maintain the energy consumption dependent on the demanded cooling capacity but to maintain a certain level of speed if the compressor is operable for maintaining a reasonable level of compressor efficiency.
It is particularly advantageous if the controller in said low operational stage maintains said speed of said compressor essentially constant and varies the interruption interval, e.g. the intervals within which the compressor is switched on or off so that the cooling power is only controlled by controlling the interruption intervals.
In such an embodiment it is of particular advantage if in said lowest operational mode said speed of said compressor is in the dimension of the minimum possible speed for the compressor. This means that the compressor is run at the lowest allowable speed for proper operation and that if only cooling power is needed which is lower than the cooling power provided at that minimum speed a further reduction is performed by interrupting the compressor.
In connection with the aforementioned explanations of various embodiments of the present invention it has not been defined how the controller determines the cooling power demanded.
One manner of determining the cooling power demanded would be to only detect the temperature present within said transport volume and to reduce the speed of the compressor to the lowest possible level.
A more advantageous manner of determining the cooling power demanded is to compare the temperature present within that transport volume and the temperature of ambient air.
With respect to the temperature detection within the transport volume the location of detection has not been defined in connection with the explanation of the aforementioned embodiments.
Generally, the temperature within the transport volume can be detected anywhere therein.
For obtaining a fast response of the temperature detection it is advantageous if the controller senses the temperature in a stream of air circulating within said transport volume because in such a case the controller obtains the proper temperature values with a short response time.
In addition, it is advantageous to sense the temperature within said transport volume close to said evaporator because in this case the cooling power demanded can be determined more precise.
In general, the controller could start in the uppermost operational stage or in the lowermost operational stage and follow the sequence of operational stages until the desired temperature is obtained.
To be able to respond precisely to temperature changes it is of advantage if the controller selects the currently necessary operational stage in accordance with the cooling power demanded.
In accordance with the present invention, as discussed above, it would be possible to have a varying compressor speed related cooling capacity of said closed cooling circuit within at least one of said upper operational stages, however, for designing an easily controllable system it is of advantage if said compressor speed related cooling capacity of said closed cooling circuit is constant within each of said upper operational stages.
With respect to the lowest operational stage the compressor speed related cooling capacity could vary too. However, it is also of advantage if said compressor speed related cooling capacity of said closed cooling circuit is constant within said lowest operational stage.
With respect to a cost effective design of the inventive cooling unit it turned out to be advantageous for said compressor speed related cooling capacity of said closed cooling circuit to be the same as the compressor speed related cooling capacity in said one of said upper operational stages covering the lowest range of cooling power of said sequence of upper operational stages.
If the controller has the possibility to switch from one upper operational stage to another operational stage such a switching is advantageously defined by a respective cooling power. To avoid at this respective cooling power a fast witching back and forth between one upper operational stage and the other operational stage it is advantageous if the controller switches from one upper operational stage to another upper operational stage with a hysteresis with respect to the level of cooling power, which means that the cooling power at which the controller switches from one upper operational stage to the next higher operational stage is higher than the cooling power at which the controller switches from the higher operational stage to the next lower operational stage.
In the course of such a switching from one operational stage to the next operational stage the cooling power provided by the closed cooling circuit could come out of control.
This is avoided if in the course of a transition from one of said upper operational stages to another of said upper operational stages said controller maintains full control of the cooling power provided by said closed cooling circuit by adjusting the speed of said compressor in accordance with a change of the compressor speed related cooling capacity.
This means that even in the course of a transition from one operational stage to the next operational stage, which has the consequence that the corresponding compressor speed related cooling capacity changes, precise control of the cooling power provided is still maintained due to the fact that the controller even in the course of such a transition is still able to adjust the cooling power by adjusting the speed of the compressor.
An advantageous embodiment of the present invention provides a compressor designed as a multi-stage compressor which is operable in a first mode using a reduced number of stages and in a second mode using all stages of said compressor for compressing refrigerant. Such a design has the advantage that when operating the compressor at a reduced number of stages the compressor speed related cooling capacity can be reduced and in addition the energy consumption is reduced due to the lower amount of energy which is needed for operating such a multi-stage compressor in a reduced number of stages.
It is of particular advantage if such a multi-stage compressor is controllable by said controller of said closed cooling circuit so as to operate in said first mode or said second mode.
It is of particular advantage according to the present invention if in one of said upper operational stages said compressor operates in said first mode and in another of said upper operational stages said compressor operates in said second mode because then different operational stages can be defined by operating the compressor in different modes, e.g. a first and a second mode, and the controller can be used to switch the compressor between said first mode and said second mode.
In an embodiment of particular advantage it is provided that said controller changes from a stage in which the compressor operates in said first mode to the stage in which the compressor operates in said second mode at a defined level of cooling power which is higher then the defined level of cooling power at which the controller switches from the operational stage in which the compressor operates in said second mode to the operational stage in which the compressor operates in said first mode. Such a hysteresis used for changing between two operational stages is advantageous insofar as it prevents the controller at a certain level of cooling power from switching back and forth between the operational stages and therefore providing an unstable controlling characteristic which in particular has the consequence that the tolerances with respect to the defined temperature within the transport volume increase.
In another advantageous embodiment according to the present invention an economizer is provided in said closed cooling circuit.
Such a economizer could be designed to be fully operable within the entire operational range of the cooling unit.
However, it is of particular advantage if said economizer can be switched by said controller between an economizer off-mode and an economizer on-mode.
For providing different compressor speed related cooling capacities it is of particular advantage if in one of said upper operational stages the closed cooling circuit is controlled to operate in an economizer off-mode and in another of said upper operational stages said closed cooling circuit is controlled to operate in an economizer on-mode. Such an embodiment of the present invention has the advantage that within the same range of speed of the compressor, different compressor speed related cooling capacities can be obtained and these different compressor speed related cooling capacities also result in a different energy consumption by the compressor, because in the economizer on-mode the energy consumption of the compressor is increased with respect to the economizer off-mode.
To avoid an unstable behaviour of the cooling unit and, therefore, to avoid increased temperature tolerances due to unstable conditions an advantageous embodiment provides that said controller switches from the operational stage in which the closed cooling circuit is in the economizer off-mode to the operational stage in which the closed cooling circuit is in the economizer on-mode at a defined level of cooling power which is higher than the defined level of cooling power at which the controller switches from the operational stage in which the closed cooling circuit is in the economizer on-mode to the operational stage in which the closed cooling circuit is in the economizer off-mode.
The aforementioned object is further achieved by a refrigerated container comprising a thermally insulated housing enclosing a transport volume to be cooled, a cooling unit for cooling air circulating in said transport cooling volume, wherein said cooling unit is designed according to the features of the various embodiments as explained before.
Further advantages of the present invention are the subject matter of the detailed description of one embodiment of the present invention.